In order to produce a relatively large product by plastic-working a metal or alloy workpiece material, it is necessary to increase the size of the workpiece material before plastic working.
A casting method has been a mainstream method for producing a large material from a light alloy such as a magnesium alloy and an aluminum alloy. However, a workpiece material produced by the casting method has a coarse crystal structure and a low strength. Accordingly, a product obtained by forging a workpiece material produced by the casting method does not have a satisfactory strength.
An example of a method for producing a billet-shaped workpiece material is a method for forging a bar-shaped body into a large-diameter body by a swaging machine. For example, Japanese Patent Publication No. H08-3675 of unexamined applications discloses forging of an aluminum alloy at a swaging ratio of 10 to 50%. Japanese Patent Publication No. 2006-152401 of unexamined applications discloses production of a magnesium alloy molded body by forging a high Al content magnesium alloy material.
In order to perform swaging normally without causing buckling of a material and the like, the ratio L/D of the length (L) to the diameter (D) of a material before swaging is 2 or less. Since the material is only slightly plastically deformed by a swaging process, the crystal structure of the material does not become so fine and the strength of the material is not improved sufficiently.
Extruding a cast product makes the crystal structure fine, whereby the extruded material has a high strength. For example, Japanese Patent Publication No. 2003-313646 discloses extrusion of an Mg—Mn-based alloy to obtain fine crystal grains and a high strength.
In an extrusion process, the strength increases as the extrusion ratio rises. In order to obtain a desired high strength by the extrusion process, the extrusion ratio (the ratio of the cross-sectional area of a material before the extrusion process to the cross-sectional area of the material after the extrusion process) needs to be, for example, 25 or more.
For example, in order to obtain a large billet of 150 mm in diameter by the extrusion process with an extrusion ratio of 25, a material before the extrusion process needs to have a diameter of 750 mm. In this case, the press capability of as high as 12,000 to 18,000 tons is required empirically, although it depends on the kind of the material. However, it is practically impossible to implement such high press capability. It has been difficult to obtain a large material with a high strength and a large diameter by the extrusion process.
When powder is used as a starting material, a billet may be produced as a workpiece material by compacting and solidifying the powder and extruding the resultant powder compact. In this case as well, the extrusion process has the same problems as those described above.
It has been difficult to produce a high strength workpiece material (billet) having a fine crystal structure while having a large diameter by any conventional methods.